Single Event Effects in microelectronic circuits Author: Klemen Koselj Advisor: Prof. Dr. Peter Križan

Agenda Introduction – what are Single Event Effects (SEE) and their classification. Ionizing radiation environment and SEE's. How SEE testing is done? –SEE testing with radiation method –SEE testing with pulsed laser method Conclusion

Microelectronic circuits in radiation environment The effects resulting from the interaction of high-energy ionizing radiation with semiconductor material can have a major impact on the performance of space-based and accelerator-based microelectronic circuitry. Two categories: –Total ionizing dose effects. –Single Event Effects (SEE).

What is a SEE? Electrical disturbance in a microelectronic circuit caused by the passage of a single ionizing particle through semiconductor material. As a single high-energy particle penetrates a circuit, it leaves behind a dense plasma track in the form of electron-hole pairs. A circuit functional error, or even a circuit failure, will occur if sufficient charge from the plasma track is collected at a sensitive circuit node.

Types of SEE Single-event upset (SEU) is a change of state or transient induced by an ionizing particle such as a cosmic ray or proton in a device. Single-event latch up (SEL) is a potentially destructive condition involving parasitic circuit elements. Other types (Single event burnout (SEB) – destructive form of SEL,...)

Types of SEE - again Categorization of SEE's is also possible in terms of whether they are soft or hard errors regarding the amount and permanency of damage made to the device. Soft errors are nondestructive. They may appear as a bit flip in a memory cell or as transients occurring on the output of an I/O, logic, or other support circuit. SEU is a soft error. Hard errors may be (but are not necessarily) physically destructive and are permanent functional effects. SEL and SEB are hard errors.

Ionizing radiation environment and SEE’s Most problems in microelectronic circuits by present date were observed in space-based electronics. Problems in operating due to SEE’s were also observed in avionics electronics. Read-out electronics in accelerator environment is affected by high-energy ionizing radiation. SEE’s were observed and are significant in a population of humans with implantable cardioverter defibrillators. SEE’s in this case are caused by secondary cosmic ray neutron flux.

SEE errors from the UoSAT-3 spacecraft in polar orbit Most SEE errors occur in the so-called South Atlantic Anomaly Errors occur because of protons with broad energy spectrum (energies from keV to several hundreds of MeV) are trapped in the so-called Van Allen belt. Significant number of errors occur at high latitudes due to galactic cosmic rays.

How does a SEE appear? Caused by a deposition of a large amount of energy in a small sensitive volume, typically 10 MeV energy deposition over 1  m particle path length. Following Bethe and Bloch the average energy loss dE per length dx is given by

Energy deposition For highly relativistic Z = 1 particles with  ~ 1, dE/dx=4.6 MeV/cm in silicon. Not enough for SEE. For a SEE to occur slow particles (  ~ ) or particles with high Z (or a combination of both parameters) are needed in order to produce 10 MeV energy deposition over 1  m which is needed to produce enough charge for SEE. Protons can not cause SEE directly. Electrons either.

Electron-hole pair creation

Where do particles with proper energy deposition come from? Spallation reactions of neutrons and protons with silicon 28 Si(n,  ) 25 Mg, 28 Si(n,p) 28 Al, and 28 Si(p,2p) 28 Al all produces recoilling heavy nuclei. Production of silicon recoil nuclei through electromagnetic interaction. 10 B(n,  ) 7 Li produces an alpha particle and the residual 7 Li. (Boron is often used for doping in semiconductor industry.)

Electron hole-pair distribution The electron-hole pair distribution depends on radial distance from the center of the track. Once the electron-hole pairs are established in the track, the carriers can be collected at junctions in the structure. Complicated drift, diffusion and recombination processes are responsible for generated charge transport. Initial electron-hole density as a function of radius from the center of ion track for various depths for (a) 70 MeV and 250 MeV Cu ions

Calculating SEE's error rates In the burst generation rate (BGR) model a SEE may occur when a high-energy particle strikes the reversed biased pn junction of a memory cell and deposits sufficient (critical) charge in small enough (sensitive) volume to cause a change in memory state. The soft error rate (SER) is given by:

Burst generation rate method To obtain quantitative measure for soft error rate we need to identify all important interactions of ionizing radiation for a given environment. Then Qc has to be estimated. In memory cells, where charge is used to store information (DRAM's and CCD's), it is assumed that a sudden spontaneous 20 percent variation in charge may cause the device to invert (from strored '1' to '0'). Finally we have to measure the fluxes and spectra of radiation in environments where soft error rate is of interest, and together with measured or calculated burst generation rate calculate particle-induced error for each of the important interactions.

Chip errors induced by sea-level cosmic rays obtained with BGR SEE rates in implantable cardioverter defibrillators were also estimated using BGR as 4.5  upset/bit-hr which is well in accordance with observations in the field.

Single event effects testing There are two important SEE testing techniques nowadays: –The tests are traditionally performed using energetic particles produced at accelerators to simulate the radiation environment in which device under test will operate. –Recently laser pulses have been used to induce SEE's.

SEE testing with the radiation method Particle accelerator testing is the standard method used to characterize the sensitivity of microelectronic circuitry to SEE. The goal of SEE testing with radiation method is to determine the cross section vs. the deposited energy (known as Linear Energy Transfer (LET))

Single event effects testing with pulsed laser method Pulsed laser method is based on the ability of laser pulses to provide a reasonable approximation of the interaction between a high- energy particle and a semiconductor. It provides complementary information and some unique characteristics and capabilities that are particularly useful for SEE studies – spatial distribution.

Benefits of pulsed laser method The laser can be focused down and imaged to a small spot (  1  m). Therefore sensitivity of individual circuit elements can be measured. This is not easily accomplished with radiation method. As long as the laser intensity is below the threshold for melting in the semiconductor, there is no permanent damage to the material. There is no ionizing radiation threat, no vacuum is required and laser tests are relatively inexpensive compared to radiation tests.

Conclusion Interaction of high-energy ionizing radiation with semiconductor material impacts the performance of microelectronic circuitry operating in space or accelerator environment. Effects of interactions with single high-energy ionizing particles causes errors in circuit operation called Single Event Effects - SEE. These errors can cause temporal or permanent damage to microelectronic circuits. Two techniques were presented, pulsed laser and radiation method, both intended to explore and characterize the SEE behavior in microelectronics circuits.